In conventional variable air volume (VAV) systems which are capable of variable output by varying the fan speed, positioning the inlet guide vanes or by controlling the pitch of an axial fan, fan control is achieved through the use of a single pressure sensor which is located somewhere in the main duct line. The pressure sensor sends an input directly to the fan control, such as a variable speed drive of an air handler unit, to cause the speed to increase, decrease or remain constant. Thus, if the pressure at the sensor location is too high, a "decrease" signal is sent and, if it is too low, an "increase" signal is sent. The location of the sensor is carefully chosen in order that the pressure sensed is most closely representative of the overall system pressure distribution over the whole load range. Ordinarily, the sensor location chosen is about two thirds of the way down the main duct as measured by distance or total pressure drop. The space temperatures are not used to control fan speed. Instead, the sensed pressure is fed back to the speed changer through a parallel summing junction/function generator control process. The speed changes alter fan speed and thereby fan outlet pressure. There is a pressure loss in the remaining main/common duct downstream of the pressure sensor. The duct system splits up the flow, losing more pressure in the branches, and the air is delivered to a VAV terminal control loop which imposes a controlled pressure loss and ultimately influences the space temperature. Thus, although temperature is the variable to be controlled, all of the controls are pressure responsive, and a single pressure sensor is used. So, even if the pressure sensor is located at the point where the sensor will give the "best" representation of the overall duct pressure distribution at design flow, when there is off .design flow and when the VAV terminals on one side of the duct system close off, however, the "best" spot changes to another location. As a result, the sensor may be inaccurate in its representation of duct pressure distribution under off-design conditions as well as those due to normal changes during the day.
Additionally, the sensed pressure provides an indirect communication between the air handler and the terminals in the sense that the air handler is operated in response to the sensed pressure and the sensed pressure is influenced by the terminals as they open and close to control the air flow and thereby the temperature in the zones. Since temperature data is not directly communicated, the operation of the system can be in an undesirable manner. For example, ventilation requirements in a zone may cause it to be overheated or overcooled.
Although the interior zones of a building are essentially isolated from seasonal temperature changes, sunload, etc., there are still variables in the cooling load due to people leaving their work areas and/or congregating for meetings, etc. Also, the cooling load is dependent upon the heat produced by lighting, equipment and personnel. Overnight, during vacations, etc., overcooling may be caused even where air is supplied at minimum ventilation requirements. As a result, it may be necessary to provide a "warmup" in the morning even though there is only a cooling load during occupancy. Similarly, it is undesirable to heat unoccupied areas such as unused offices.